Effect of annealing on properties and performance of HfO2/SiO2 optical coatings for UV-applications.

The field of ultraviolet (UV)-laser applications is currently experiencing rapid growth in the semiconductor processing, laser micromachining and biomedical markets. Key enablers for these technologies are optical coatings used to manipulate and guide laser beams in a targeted manner. As laser power, laser fluence and pulse frequencies increase, the demands on the physical properties of the coating materials become more stringent. Ion beam sputtering is a technique that allows producing optical coatings with the low losses required in these applications. In this study, we investigate the influence of ion beam sputtering (IBS) parameters on the optical properties of HfO2 and SiO2 single layers as well as the impact of annealing duration at 475 °C for anti-reflective (AR) and highly reflective (HR) optical coatings at 355 nm. For HfO2 sputtered from a metal target the O2 flow during the coating process is a key parameter to reduce absorption. SiO2 single layers exhibit improved transmission in the UV-range as the ion beam energy for the sputtering process is reduced. Furthermore, a complex behavior for film stress, absorption, surface roughness and coating structure was unraveled as a function of annealing duration for AR- and HR-coatings at 355 nm. The reflectance of the HR-mirror after optimized annealing exceeded 99.94% at 355 nm and a high laser induced damage threshold (LIDT) of 6.9 J/cm2 was measured after 2 hours of annealing. For the AR-coating a LIDT-value of 15.7 J/cm2 was observed after 12 hours of annealing.

Plasma-Enhanced Atomic Layer Deposition of HfO2 with Substrate Biasing: Thin Films for High-Reflective Mirrors.

Tuning ion energies in plasma-enhanced atomic layer deposition (PEALD) processes enables fine control over the material properties of functional coatings. The growth, structural, mechanical, and optical properties of HfO2 thin films are presented in detail toward photonic applications. The influence of the film thickness and bias value on the properties of HfO2 thin films deposited at 100 °C using tetrakis(dimethylamino)hafnium (TDMAH) and oxygen plasma using substrate biasing is systematically analyzed. The HfO2 films deposited without a substrate bias show an amorphous microstructure with a low density, low refractive index, high incorporation of residual hydroxyl (OH) content, and high residual tensile stress. The material properties of HfO2 films significantly improved at a low bias voltage due to the interaction with oxygen ions accelerated to the film. Such HfO2 films have a higher density, higher refractive index, and lower residual OH incorporation than films without bias. The mechanical stress becomes compressive depending on the bias values. Further increasing the ion energies by applying a larger substrate bias results in a decrease of the film density, refractive index, and a higher residual OH incorporation as well as crystalline inclusions. The comparable material properties of the HfO2 films have been reported using tris(dimethylamino)cyclopentadienyl hafnium (TDMACpH) in a different apparatus, indicating that this approach can be transferred to various systems and is highly versatile. Finally, the substrate biasing technique has been introduced to deposit stress-compensated, crack- and delamination-free high-reflective (HR) mirrors at 355 and 532 nm wavelengths using HfO2 and SiO2 as high and low refractive index materials, respectively. Such mirrors could not be obtained without the substrate biasing during the deposition because of the high tensile stress of HfO2, leading to cracks in thick multilayer systems. An HR mirror for 532 nm wavelength shows a high reflectance of 99.93%, a residual transmittance of ∼530 ppm, and a low absorption of ∼11 ppm, as well as low scattering losses of ∼4 ppm, high laser-induced damage threshold, low mechanical stress, and high environmental stability.

Effect of ion beam figuring and subsequent antireflective coating deposition on the surface absorption of CaF2 at 193 nm.

The surface absorption of CaF2 at 193 nm after surface treatment by ion beam figuring (IBF) and the subsequent antireflective (AR) coating process was investigated by using the laser-induced deflection technique. In comparison to a standard pitch polishing, enhanced surface absorption (subsurface damage) is observed after IBF treatment. However, after AR coating deposition, the total surface absorption of IBF treated samples does not exceed values obtained for pitch polished samples. The experimental results provide indication that IBF treatment yields both fluorine displacement and deficiency in the CaF2 subsurface region, which is annealed during the coating process by the high temperature and the fluorine supply from the fluoride coating materials, respectively.

Enhanced laser-induced deflection measurements for low absorbing highly reflecting mirrors.

A new concept enhances the capability of photo-thermal absorption measurements with transversal probe beam guiding by overcoming drawbacks such as a lack of sensitivity for materials with low photo-thermal response and/or round substrate geometry. The sandwich concept using the laser-induced deflection technique is introduced and tested for the investigation of highly reflecting (HR) coatings. The idea behind the sandwich concept is based on the decoupling of the optical materials for the pump and probe beams. This is realized by either placing a HR coated rectangular substrate in between two optical (sandwich) plates or attaching a HR coated thin round substrate onto one optical plate. For both configurations, the sandwich concept results in a strong increase in sensitivity for the measurement of HR coatings deposited onto photo-thermally insensitive substrates. Experiments reveal that for a CaF2 substrate, up to two orders of magnitude enhancement in sensitivity can be achieved.

Laser induced deflection technique for absolute thin film absorption measurement: optimized concepts and experimental results.

Using experimental results and numerical simulations, two measuring concepts of the laser induced deflection (LID) technique are introduced and optimized for absolute thin film absorption measurements from deep ultraviolet to IR wavelengths. For transparent optical coatings, a particular probe beam deflection direction allows the absorption measurement with virtually no influence of the substrate absorption, yielding improved accuracy compared to the common techniques of separating bulk and coating absorption. For high-reflection coatings, where substrate absorption contributions are negligible, a different probe beam deflection is chosen to achieve a better signal-to-noise ratio. Various experimental results for the two different measurement concepts are presented.

Nonlinear absorption in single LaF(3) and MgF(2) layers at 193 nm measured by surface sensitive laser induced deflection technique.

We report nonlinear absorption data of LaF(3) and MgF(2) single layers at 193 nm. A highly surface sensitive measurement strategy of the laser induced deflection technique is introduced and applied to measure the absorption of highly transparent thin films independently of the substrate absorption. Linear absorptions k=(alphaxlambda)/4pi of 2x10(-4) and 8.5x10(-4) (LaF(3)) and 1.8x10(-4) and 6.9x10(-4) (MgF(2)) are found. Measured two photon absorption (TPA) coefficients are beta=1x10(-4) cm/W (LaF(3)), 1.8x10(-5), and 5.8x10(-5) cm/W (MgF(2)). The TPA coefficients are several orders of magnitude higher than typical values for fluoride single crystals, which is likely to result from sequential two step absorption processes.

Characterization of low losses in optical thin films and materials.

Residual absorption in optical coatings and materials is directly measured by means of the laser-induced deflection (LID) technique. For transmissive coatings a measurement strategy is introduced that allows for the separation of different absorptions of the investigated sample (bulk, coating, surface) by use of only one sample. Laser irradiation yields absorption values between 2 x 10(-3) and 2.9 x 10(-2) for antireflecting and highly reflecting (HR) coatings at 193 nm and 30.6 x 10(-6) for a HR mirror at 527 nm. Use of laser-induced fluorescence at 193 nm excitation reveals trivalent cerium and prasodymium and hydrocarbons in different single layers and coatings. In addition to correlation with absorption data, the influence of a high fluorescence quantum yield on the absorption measurement is discussed.